This invention relates to drive circuits for fluorescent lamps. More particularly, this invention relates to fluorescent lamp power supply circuits that use a first feedback loop to regulate lamp current amplitude and a second feedback loop to synchronize direct current-to-alternating current converter circuitry with the resonant frequency of a ceramic step-up transformer with isolated voltage feedback.
Fluorescent lamps increasingly are being used to provide efficient and broad-area visible light. For example, portable computers, such as lap-top and notebook computers, use fluorescent lamps to back-light or side-light liquid crystal displays to improve the contrast or brightness of the display. Fluorescent lamps also have been used to illuminate automobile dashboards and may be used with battery-driven, emergency-exit lighting systems.
Fluorescent lamps are useful in these and other low-voltage applications because they are more efficient, and emit light over a broader area, than incandescent lamps. Particularly in applications requiring long battery life, such as portable computers, the increased efficiency of fluorescent lamps translates into extended battery life, reduced battery weight, or both.
In low-voltage applications such as those discussed above, a power supply and control circuit must be used to operate the fluorescent lamp. In many applications in which fluorescent lamps are used, a direct current (DC) source ranging from 3 to 20 volts provides power to operate the lamp. Fluorescent lamps, however, generally require alternating current (AC) voltage sources of about 1000 volts root-mean-square (V.sub.RMS) to start, and over about 200 V.sub.RMS to efficiently maintain illumination. Fluorescent lamps operate most efficiently if driven by a low-distortion sine wave. Excitation frequencies for fluorescent lamps typically range from about 20 kHz to about 100 kHz. Accordingly, a DC-AC power-supply circuit is needed to convert the available low-voltage DC input to a high-voltage, high-frequency AC output needed to power the fluorescent lamp.
FIG. 1 shows a block diagram of a previously-known fluorescent lamp power supply circuit used to convert low-voltage DC to high-voltage, high-frequency AC. The circuit of FIG. 1 is described in more detail in U.S. Pat. No. 5,548,189 to Williams (the "'189 Patent"), which is incorporated in its entirety herein by reference (the '189 Patent and this application are commonly assigned). Lamp circuit 10 includes low-voltage DC source 12, voltage regulator 14, DC-AC converter 16, fluorescent lamp 18 and amplitude feedback circuit 20. Low-voltage DC source 12 provides power for circuit 10, and may be any source of DC power. For example, in the case of a portable computer such as a lap-top or notebook computer, DC source 12 may be a nickel-cadmium or nickel-hydride battery providing 3-5 volts. Alternatively, if lamp circuit 10 is used with an automobile dashboard, DC source 12 may be a 12-14 volt automobile battery and power supply.
DC source 12 supplies low-voltage DC to voltage regulator 14, which may be a linear or switching regulator. For maximum efficiency, a switching regulator can be used. The '189 Patent describes implementing voltage regulator 14 using the LT-1072 switching regulator manufactured by Linear Technology Corporation, Milpitas, Calif. Other devices, however, could be used.
Voltage regulator 14 provides regulated low-voltage DC output V.sub.dc to DC-AC converter 16. DC-AC converter 16 converts V.sub.dc to a high-voltage, high-frequency AC output V.sub.AC of sufficient magnitude to drive fluorescent lamp 18. The peak amplitude of V.sub.AC is approximately 50-200 times greater than the amplitude of V.sub.dc. As described in the '189 Patent, fluorescent lamp 18 may be any type of fluorescent lamp. For example, in the case of lighting a display in a portable computer, fluorescent lamp 18 may be a cold- or hot-cathode fluorescent lamp.
Voltage regulator 14 and DC-AC converter 16 deliver high-voltage AC power to fluorescent lamp 18. Amplitude feedback circuit 20 generates feedback voltage AFB, which is proportional to fluorescent lamp current I.sub.LAMP. This current-mode feedback controls the output of voltage regulator 14 as a function of the magnitude of current I.sub.LAMP. The output of voltage regulator 14, in turn, controls the output of DC-AC converter 16. As a result, the magnitude of current I.sub.LAMP conducted by fluorescent lamp 18, and hence the intensity of light emitted by the lamp, is regulated to a substantially constant value.
By including fluorescent lamp 18 in a current-mode feedback loop with voltage regulator 14, the fluorescent lamp's current and light intensity are regulated and remain substantially constant despite changes in input power, lamp impedance or environmental factors. Lamp circuit 10 similarly compensates for variations in the output voltage of low-voltage DC source 12. These features extend the useful lifetime of a fluorescent lamp in some applications.
FIG. 2 shows a more detailed block diagram of previously known lamp circuit 10. In particular, converter 16 includes self-oscillating driver circuit 22 and ceramic step-up transformer 24. Self-oscillating driver circuit 22 chops the low-voltage DC signal V.sub.dc supplied by voltage regulator 14 to create a low-voltage, high-frequency square-wave AC signal V.sub.ac that is supplied to ceramic step-up transformer 24. Ceramic step-up transformer 24 operates as a highly frequency-selective, high gain step-up device, and transforms low-voltage, high-frequency AC signal V.sub.ac to high-voltage, high-frequency AC signal V.sub.AC.
FIG. 3 provides a graph of impedance versus frequency for ceramic step-up transformer 24 having a resonant frequency F.sub.R. In theory, ceramic step-up transformer 24 has zero impedance at resonant frequency F.sub.R and infinite impedance at non-resonant frequencies. Ceramic step-up transformer 24 actually has negligible impedance at resonance and high impedance at all other frequencies. Thus, as frequency is tuned towards resonant frequency F.sub.R from either direction, the impedance abruptly spikes down to its lowest value. The steep non-linear ramps on either side of the impedance spike are sometimes referred to as "skirts."
In particular, at resonance, the piezoelectric characteristics of ceramic step-up transformer 24 make the device a high gain, step-up device with negligible internal impedance. At frequencies other than resonant frequency F.sub.R, ceramic step-up transformer 24 behaves like a high-impedance circuit (theoretically approximating an open circuit). At "skirt" frequencies, ceramic step-up transformer 24 has intermediate ranges of impedance.
Ceramic step-up transformer 24 therefore functions as a highly-selective narrow-range filter. As a result, the input to ceramic step-up transformer 24 need not be substantially sinusoidal. For example, if V.sub.ac is a square-wave at resonant frequency F.sub.R, V.sub.ac may be expressed (in a Fourier series) as a sinusoid at frequency F.sub.R, plus an infinite series of sinusoids at odd-order harmonics of frequency F.sub.R. Ceramic step-up transformer 24 amplifies the sinusoidal component of V.sub.ac at F.sub.R, and attenuates the higher-frequency harmonics. Thus, ceramic step-up transformer 24 advantageously generates a low-distortion, high-voltage, high-frequency sine wave V.sub.AC at resonant frequency F.sub.R to optimally drive fluorescent lamp 18.
Circuit components that comprise self-oscillating driver circuit 22 primarily determine the driver's oscillation frequency f.sub.osc. Ideally, oscillation frequency fosc equals resonant frequency F.sub.R. As a result of component tolerances, environmental conditions and aging of driver circuit 22 and ceramic step-up transformer 24, however, oscillation frequency f.sub.osc may vary from resonant frequency F.sub.R by as much as .+-.20%. If fosc is significantly off-resonance, lamp circuit 10 of FIG. 2 may not operate efficiently, or may even fail to operate altogether.
As shown in FIG. 6 of the '189 Patent, previously-known lamp circuits have addressed off-resonance operation as a means to control the amplitude of the lamp current. FIG. 4 shows a block diagram of one previously known lamp circuit that uses a frequency control loop to maintain stable operation both on-resonance and off-resonance. In particular, lamp circuit 40 includes low-voltage DC source 12, lamp 18, ceramic step-up transformer 24, operational amplifier (opamp) 30, voltage-controlled oscillator (VCO) 32 and driver 34.
Opamp 30 has a first input 26 coupled to voltage-control signal VC provided by low-voltage DC source 12, and a second input 28 coupled to feedback signal FB from lamp 18. As described below, VC controls the output frequency of VCO 32. Opamp 30 generates a DC-voltage signal that is proportional to the difference between feedback signal FB and voltage-control signal VC, and that sets the operating frequency of VCO 32. VCO 32 generates an AC signal that is amplified by driver 34. The output of driver 34 is coupled to the input of ceramic step-up transformer 24. Ceramic step-up transformer 24 outputs a stepped-up, sinusoidal voltage waveform to drive lamp 18. Feedback signal FB is proportional to lamp current I.sub.LAMP, and is used to regulate the lamp drive.
Low-voltage DC source 12, opamp 30 and VCO 32 control the oscillation frequency of lamp circuit 40. By adjusting voltage-control signal VC, lamp circuit 40 can be directed to drive lamp 18 to resonant frequency F.sub.R of ceramic step-up transformer 24. In addition, control signal VC can be used to drive lamp 18 off-resonance, and therefore vary the magnitude of lamp current I.sub.LAMP and intensity of lamp 18.
The previously-known lamp circuit of FIG. 4 thus uses complex circuits to ensure that lamp circuit 40 can operate off-resonance without disabling the circuit or shutting down lamp 18. The circuit does not, however, provide a simple means to both control the amplitude of the lamp current and match the operating frequency of the driver to the resonant frequency of the ceramic step-up transformer.
In view of the foregoing, it would therefore be desirable to provide a ceramic step-up transformer lamp circuit and method that provides amplitude feedback control and frequency feedback control to regulate lamp current and oscillation frequency.
It further would be desirable to provide a ceramic step-up transformer lamp circuit and method that regulates lamp current and oscillation frequency with minimal complexity.